SRS-A (Slow Reacting Substance of Anaphylaxis) was first described in 1940 and shown to be released from monkey lung in 1966. Although SRS-A is well known as a mediator of bronchial asthma, the biochemical mechanism of its synthesis and the enzymes involved in its formation remain unknown. In this study a number of compounds were examined for their ability to increase the biosynthesis of SRS-A and for their possible incorporation into this biologically active compound. The relationship of SRS-A to other mediators of bronchial asthma was investigated.